A virtue of interferometric devices used for examination of the internal structure of objects by means of low-coherence optical radiation, is a potential for obtaining images of turbid media with high spatial resolution, as well as performing non-invasive diagnostics in the course of medical examination, and non-destructive examination in the course of technical diagnostics of various equipment. Such devices are fairly well-known (e.g., by U.S. Pat. Nos. 5,321,501; 5,383,467; 5,459,570; 5,582,171; 6,134,003; and 6,657,727). The optical scheme of an interferometric device may be fully or partially implemented using bulk optical elements (U.S. Pat. No. 5,383,467), but more often such devices are designed using fiber-optic elements (U.S. Pat. Nos. 5,321,501; 5,459,570; 5,582,171; and 6,847,453).
Traditionally, such devices include a source of low coherence optical radiation, an optical interferometer and a delivering member for delivering low coherence optical radiation to the object under study. The optical interferometer includes at least one optical coupler and two arms. At least one of the arms of the optical interferometer includes a controlled scanner which performs in-depth scanning of the coherence window of low coherence optical radiation within the observation range. The controlled scanner is connected with a source of control voltage. The delivering member for delivering low-coherence optical radiation to the object under study may be part of one of the arms of the optical interferometer (e.g., U.S. Pat. No. 5,321,501; RU Patent No. 2100787), or may be placed outside the optical interferometer (e.g., U.S. Pat. No. 6,847,453). No matter where the delivering member for delivering low coherence optical radiation to the object under study is placed, the optical interferometer is usually implemented as a Michelson interferometer (e.g., U.S. Pat. Nos. 5,321,501; 5,459,570; 6,069,698; and 6,134,003; RU Patent No. 2100787; RU Patent No. 2148378; and RU Patent No. 2169347), or as a Mach-Zehnder interferometer (e.g., U.S. Pat. Nos. 5,582,171; and 6,687,010). Other known embodiments use optical interferometers of a hybrid type (e.g., U.S. Pat. Nos. 5,291,267; and 6,657,727).
In interferometric devices that employ the optical coherence tomography method, an interference signal is only registered, when the difference between optical lengths of the arms of the optical interferometer is within the coherence window dimension, which ranges from 5 to 20 micron. While examining an object, in-depth scanning of the object under study is achieved by scanning the coherence window within an observation range, which typically is up to ˜2-5 mm. Thus, when designing an interferometric device according to any of the above optical schemes; special attention needs to be paid for matching optical lengths of the arms of the optical interferometer.
Matching between the optical arms of the optical interferometer is capable of being disrupted in the course of device operation, for example, as a result of replacing the optical probe, of unequal change in the optical lengths of the arms of the optical interferometer due to a thermal impact of the environment, of lateral scanning of objects with deep profiles, of moving the object under study or the lateral scanner. In addition, in the course of device operation, there may be a need to shift the observation range with a purpose of examining other parts of the object.
There is also known including an additional controlled scanner in one of the arms of the optical interferometer in order to control the location of a boundary of the observation range (e.g., U.S. Pat. Nos. 6,069,698; 6,191,862; 6,552,796; and 6,615,072).
A disadvantage of these devices is that the controlled scanners (both the controlled scanners performing in-depth scanning of the coherence window of low coherence optical radiation within the observation range, as well as the controlled scanners that perform changing of a boundary location of the observation range), are implemented as mechanical delay lines. While it is feasible to automate the measurement process, devices using such delay lines are expensive, fairly complicated, sensitive to vibration and environment conditions, and therefore are hardly suitable for medical examinations, especially in vivo studies.
Another shortcoming of known technical solutions, is their inability to correct distortion of the rendered tomographic image of the cross-section of the object under study in the case when the interferometric device is part of an optical coherence tomography device, wherein the lateral scanning device is, at least partially, part of the optical probe. This distortion is experienced due to aberrations in the optical path length for the low coherence optical radiation directed towards the object under study, caused by lateral scanning. The distortion becomes particularly obvious when an object with a flat surface is examined. As a result of this aberration, the rendered tomographic image of the cross-section of the object under study appears curved. The distortion of the tomographic image is associated with tomographic image rendering from an interference signal, the latter being a result of interference of optical radiation returning from the object under study and optical radiation passing along a reference path. For the lateral scanning method mentioned above, the low coherence optical radiation is directed towards the object under study from points which are located at different distances from the optical axis of the device. Thus, while the optical path length for the low coherence optical radiation propagating along a reference path is invariable, the optical path length for the low coherence optical radiation directed towards the object under study does not remain constant in the process of lateral scanning, which overall results in distortion of the rendered tomographic image.
The international application PCT/RU 2003/000252 (RU Patent No. 2242710) describes an interferometric device which provides correction of the tomographic image distortion by correcting the above-mentioned aberration with a specific lens system installed inside of the fiber-optic probe. However in some cases, for instance, for medical examinations the required dimensions of the fiber-optic probe are so diminutive that would impede introduction of such a lens system.
Two modifications of an interferometric device are known from RU Patent No. 2100787. One of them includes a source of low coherence optical radiation and an optical interferometer, which are optically coupled. The optical interferometer is implemented as a Michelson interferometer and includes an optical coupler, a sampling arm and a reference arm. The sampling arm includes a delivering member for delivering low coherence optical radiation to the object under study, which is designed an optical probe.
One of the arms of the optical interferometer includes a fiber-optic controlled scanner, which performs a function of in-depth scanning of a coherence window of the low-coherence optical radiation within an observation range of the object under study. The fiber-optic controlled scanner is designed as a fiber-optic piezoelectric controlled delay line which allows for varying the optical path length of a corresponding arm of the optical interferometer within at least several tens of operating wavelengths of the source of optical radiation. The fiber-optic controlled scanner includes a piezoelectric element characterized by a high transverse inverse piezoelectric effect and an optical fiber attached to the piezoelectric element. The piezoelectric element is capable of having an electric field produced within it. A dimension of the piezoelectric element in a direction generally orthogonal to the electric field vector substantially exceeds a dimension of the piezoelectric element in a direction generally aligned with the electric field vector. The length of the optical fiber substantially exceeds a diameter of the piezoelectric element. The fiber-optic controlled scanner is connected with a source of AC control voltage.
The second modification of the interferometric device known from RU Patent No. 2100787 also includes a source of low-coherence optical radiation and an optical interferometer, which are optically coupled. The optical interferometer is implemented as a Michelson interferometer and includes an optical coupler, a sampling arm and a reference arm. The sampling arm includes a delivering member for delivering low-coherence optical radiation to the object under study, which is designed an optical probe. Each arm of the optical interferometer includes a fiber-optic controlled scanner which performs a function of in-depth scanning of a coherence window of the low-coherence optical radiation within an observation range of the object under study. Each fiber-optic controlled scanner is designed as a fiber-optic piezoelectric controlled delay line. The piezoelectric fiber optic delay line allows for varying the optical path length of a corresponding arm of the optical interferometer within at least several tens of operating wavelengths of the source of optical radiation. The delay line includes a piezoelectric element characterized by a high transverse inverse piezoelectric effect and an optical fiber attached to the piezoelectric element. The piezoelectric element is capable of having an electric field produced within it. A dimension of the piezoelectric element in a direction generally orthogonal to the electric field vector substantially exceeds a dimension of the piezoelectric element in the direction generally aligned with the electric field vector. The length of the optical fiber substantially exceeds a diameter of the piezoelectric element. The optical interferometer further includes a source of AC control voltage connected with the fiber optic controlled scanner.
According to RU Patent No. 2100787, both modifications of the interferometric device may be used as part of a device for optical coherence tomography.
The interferometric devices known from RU Patent No. 2100787 share one major advantage—they may be implemented entirely fiber-optic without using expensive moving mechanical parts. This facilitates application of such devices in medical practice, since fiber-optic controlled scanners used in the devices do not require additional adjustment and calibration in the course of operation. However, the interferometric devices known from RU Patent No. 2100787, do not provide means for controlling a boundary location of the observation range. In addition, these devices do not allow for a distortion correction of the tomographic image of the cross-section of the object under study. The distortion is caused by the optical path length aberration for the low-coherence optical radiation directed towards the object under study. Such aberration occurs when an interferometric device is part of a device for optical coherence tomography, wherein the means for lateral scanning is, at least partially, part of the optical probe.